In the field of this invention it is known that in a simple load driver circuit a differential amplifier is used to amplify a voltage across a current sense resistor carrying a load current.
Referring to FIG. 1, such a circuit is shown having an input node 10 and a ground node 20, between which there is an input voltage VIN.
A high side current sense resistor 30 has a first terminal coupled to the input node 10, and a second terminal to be further described below.
An amplifier 40, which is a typical differential amplifier, has an inverting input coupled to the input node 10, a non-inverting input coupled to the second terminal of the resistor 30 and an output coupled to an output node 50.
In operation, a voltage V1 is developed across the resistor 30. The voltage developed is proportional to the current ILOAD flowing in the resistor 30. Ideally, the voltage V1 should be insignificant compared to the input voltage VIN. A voltage V2 between the output node 50 and the ground node 20 is defined by:V2=A(V+−V−)  Equation 1
Where A is the gain of the amplifier 40, V+ is the voltage at the non-inverting input of the amplifier 40 and V− is the voltage at the inverting input of the amplifier 40.
In the circuit shown, this may be written as:V2=A*V1  Equation 2
This circuit works well in theory, and in practice at low frequencies. However, at high frequencies, the amplifier 40 will typically have a low Common Mode Rejection Ratio (CMRR), and the output equation must be re-written to take this into account:V2=A(V1+VCM/CMRR)  Equation 3
where VCM is any high frequency variation in the input voltage VIN. If the CMRR is low, and the common mode voltage VCM is large with respect to V1, then any variation in VIN will cause a large variation at V2.
This is undesirable since the circuit is typically required to have an output equivalent to equation 1 above (namely an output which is linearly proportional to the circuit input voltage) over a large frequency range.
The problem has been addressed in a variety of ways. A standard “instrumentation amplifier” configuration uses three differential amplifiers and does not require precision components. However, an “instrumentation amplifier” arrangement requires higher precision components than are used in this invention. Furthermore, this invention may provide a better CMRR figure than an instrumentation amplifier when a high frequency common mode signal is applied.
EP1176711 A2 discloses a scheme for improving harmonic distortion within a differential amplifier.
U.S. Pat. No. 6,218,901 B1 and U.S. Pat. No. 6,429,700 B1 disclose methods of setting an output common mode voltage of an amplifier with a differential output.
A need therefore exists for an amplifier arrangement, circuit and method with improved CMRR wherein the abovementioned disadvantages may be alleviated.